Modern printing and die cutting processes commonly include the step preparing a printing or die cutting plate through acid etching of the plate. In preparation of a printing plate, an image of text or graphics is acid etched into a photopolymer layer disposed over a rectangular piece of flexible sheet steel. In preparation of a die cutting plate, acid directly etches away the plate material, leaving cutting ridges in desired locations. Sheet steel is a preferred die cutting plate material because it is durable, is easily etched, and is ferromagnetic. Sheet steel is a preferred photopolymer printing plate substrate material because it is durable and is ferromagnetic. Such plates commonly are wrapped around and fixedly attached to the exterior radial surface of the print or die cutting roller of a roller press. commonly, the means for attaching such plates to such rollers is a multiplicity of magnets embedded within the exterior radial surface of the roller.
In order to achieve a secure magnetic bond between a flexible steel printing or die cutting plate and the exterior radial surface of a roller, it is desireable to orient permanent magnets embedded within the exterior radial surface of the roller so that they create a multiplicity of zones of high magnetic flux density. A known method of doing so utilizes a plurality of substantially rectangular oblong magnet receiving channels milled into the exterior radial surface of the roller, the longitudinal midlines of the channels being parallel with the axis of rotation of the roller. Within each channel, a multiplicity of permanent magnets are fixedly mounted and aligned so that their polar axes are parallel with the longitudinal midlines of the channels. The magnets are arranged so that their poles are in a NN-SS-NN-SS-NN configuration, and magnetically soft steel pole pieces are interspaced between each of the magnets. The alternating pole configuration of the series of magnets causes magnetic induction to occur within the pole pieces, resulting in heightened concentrations of lines of magnetic flux emanating from and entering into the outer walls of the pole pieces. The depth of the magnet receiving channels within the roller, the height and curvature of the permanent magnets, and height and curvature of the pole pieces are fitted so that the outer walls of the magnets and the pole pieces form smooth continuations of the exterior radial surface of the roller.
Such a configuration of magnet receiving channels, magnets and pole pieces results in a multiplicity of zones of high concentration of magnetic flux lines on the exterior radial surface of the roller. When such a steel printing or die cutting plate is wrapped over the exterior radial surface of such a roller, each magnet embedded within the roller is armatured; each armature consisting of a north pole piece in contact with the magnet's north pole, a south pole piece in contact with the magnet's south pole; and the ferromagnetic printing or die cutting plate contacting and spanning between the north and south pole pieces. Allowing the plate to act as an armaturing link between the north and south poles of each magnet results in a strong magnetic bond between the plate and the roller.
A channeled roller such as is described above preferably is composed of a highly durable material such as steel. However, ordinary steel is ferromagnetic. If the above described magnet and pole piece series were to be installed directly into a channel milled into a magnetic steel roller, the roller itself would serve as an armaturing link between the pole pieces. Magnetic flux density within a magnetic armature is proportional to the cross sectional area of the armature. Allowing an underlying steel roller to act as an additional armaturing link increases the cross-sectional area of the armature, decreasing flux density within the overlying printing or die cutting plate. Thus, use of a magnetic steel channeled roller may cause the attractive force between the roller and the plate to be unacceptably low. A known method of solving this problem is to form the roller by milling channels into a non-magnetic stainless steel roll. Non-magnetic stainless steel contacting the north and south pole pieces will not act as an armaturing link between the poles. However, utilization of stainless steel as the roller substrate material is undesirable because stainless steel is much more expensive than common magnetic steel.
The instant inventive magnetic roller provides the dual benefits of strength and economy of magnetic steel rollers, while preserving an armaturing path passing primarily through the steel print or die cutting plate. Such benefits are achieved by providing non-magnetic spacers mounted between the magnet/pole piece series and an underlying channeled roller composed of magnetic steel.